The present invention is directed to durable magnetorheological (MR) fluids containing at least one lubricative colloidal additive. The lubricative colloidal additives provide the MR fluids of the present invention with superior anti-wear and anti-friction properties. It is preferred that the lubricative colloidal additive includes colloidal polytetrafluoroethylene, colloidal graphite, colloidal molybdenum disulfide, or mixtures thereof. The MR fluids of the present invention are comprised of mechanically hard magnetizable particles, a carrier fluid derived from a polyalphaolefin and a plasticizer, and at least one colloidal additive.
Magnetorheological (MR) fluids are substances that exhibit the rather unique property of being able to reversibly change their apparent viscosity through the application of a magnetic field. For a MR fluid, the apparent viscosity, and related flow characteristics of the fluid, can be varied by controlling the applied magnetic field. Such fluids have wide application in vibration dampening devices such as, for example, shock absorbers, vibration dampers, force/torque transfer (clutch) devices, and the like, and especially in systems in which variable control of the applied dampening/force is desirable.
MR fluids are generally suspensions of finely divided magnetizable particles in a base carrier liquid. The particles are typically selected from iron, nickel, cobalt, and their magnetizable alloys. The base carrier liquid is generally a mineral oil, synthetic hydrocarbon, water, silicone oil, esterified fatty acid or other suitable organic liquid. MR fluid generally further contains a thickener or thixotropic agent to control settling, a phosphorus additive to inhibit wear and an organomolybdenum additive to inhibit friction. A surfactant may also be added to promote dispersability of the particulates in the suspension.
The anti-wear additives used in prior art MR fluids have generally been selected from well-known anti-wear additives used in, for example, engine lubricants. These include thiophosphorus additives such as zinc dialkyl dithiophosphate (ZDDP). U.S. Pat. No. 5,683,615, for instance, describes a MR fluid comprising magnetic-responsive particles, a carrier fluid and at least one thiophosphorus or thiocarbamate; and U.S. Pat. No. 5,906,767, describes a MR fluid comprising magnetic-responsive particles, a carrier fluid and at least one phosphorus additive. Neither patent, however, discloses or suggests the use of colloidal additives in a MR fluid to inhibit wear.
The anti-friction additives used in prior art MR fluids have also been generally selected from well-known organomolybdenum compounds used as anti-friction additives in engine lubricants. For example, U.S. Pat. No. 5,705,085 describes a MR fluid that includes magnetic-responsive particles, a carrier fluid and an organomolybdenum; and U.S. Pat. No. 5,683,615 also describes the use of the same organomolybdenums in the MR fluids disclosed. Neither patent, however, discloses or suggests the use of colloidal additives in a MR fluid to inhibit friction.
The anti-wear and anti-friction additives commonly used in prior art MR fluids such as ZDDP and organomolybdenum, however, pose environmental hazards due to the presence of heavy metals. More environmentally friendly anti-wear and anti-friction additives would be preferred, but as yet, have not been found.
The present invention is directed to providing MR fluids that contain colloidal additives that will substantially replace the phosphorus-based anti-wear additives and organomolybdenum anti-friction additives typically found in prior art MR fluids.
The present invention is directed to durable MR fluid formulations comprising mechanically hard magnetizable particles, a carrier fluid derived from a polyalphaolefin and a plasticizer, and at least one colloidal additive. It has been found that durable MR fluids can be formulated that have anti-wear and anti-friction properties, but do not contain heavy metal components which are known to pose environmental hazards. The MR fluids of the present invention include at least one colloidal additive that imparts such anti-wear and anti-friction properties. While it is contemplated that any suitable colloidal additive that imparts anti-wear and anti-friction properties to the MR fluid may be used, it is preferred that the colloidal additive be selected from colloidal polytetrafluoroethylene, colloidal graphite, colloidal molybdenum disulfide, and mixtures thereof. Table 1, as follows, shows the performance of representative colloidal materials in a base oil compared against a base oil containing no additives. Results obtained from a manufacturer of such colloidal additives, (Acheson Colloids Company, Port Huron, Mich.) show that anti-wear and anti-friction properties improve with the use of such colloidal additives as compared with the base oil with no additive.
The colloidal additives, as contemplated for use in the durable MR fluids of the present invention, inhibit surface-to-surface contact and scuffing while providing reliable lubrication under boundary lubrication conditions.